1. Field of the Invention
The present invention relates to a recording medium for thermal transfer printers. More particularly, the invention relates to the preparation of a recording medium having clear image quality and good preservation.
2. Description of the Related Art
A thermal transfer method has been widely used in office automation (OA) apparatuses such as facsimiles or printers. Recently, this method is applied even to the preparation of identification cards and the printing of electrical images from cameras, and thus, is an important means of photo printers together with methods for inkjet printing.
The thermal transfer method may be roughly divided into a sublimation type and a melt type. In the sublimation type, a transfer layer of recording medium consists of a thermal sublimation dye and a binder resin. Dye is delivered to a card or an accepting paper in proportion to thermal energy applied by a thermal element to form an image. This method is also referred to as a dye diffusion type. Since the amount of dye transferred is controlled in proportion to the applied thermal energy, continuous density gradation can be easily achieved on the transferred image. Color films having yellow, magenta, cyan colors, and the like belong to this class.
In the melt type, a transfer layer of recording medium has a thermal melting property. Heat applied by a thermal element to the transfer layer melts and transfers the transfer layer to a card or an accepting paper and then is solidified again. In the melt type, the entire resin of a coating layer is transferred to the card or accepting paper. Black or image protecting films belong to this class.
To obtain photographs or color images, there is a need for a thermal transfer recording ribbon which provides color and a recording medium which realizes color. The thermal transfer recording ribbon is prepared as described above. For the recording medium, plastic cards made of polyvinyl chloride and the like, and films or synthetic papers made of polyethyleneterephthalate, polyethylene, polypropylene, polyester, and the like, are used. The base of the photographic recording medium is primarily polyethyleneterephthalate or polypropylene.
The base used as a photographic recording medium should, first of all, be able to completely realize the color image to be transferred. Polyethyleneterephthalate is primarily used as the base for the recording medium and has better thermal stability, whiteness and smoothness than polypropylene. Unfortunately, polyethyleneterephthalate has a relatively small contact area with a recording ribbon by a heating head, and thus has insufficient dye transfer properties. As a result, printing stains are produced, thereby failing to obtain uniform image. This phenomenon is particularly serious in high speed printers. Generally, non-uniform image makes preservation of photograph against outer environment poor, which brings about many problems.